I'm assuming that all I need to do is turn off the heat and wait for things to cool off before I mess with these pipes. Is that correct?

++ That is correct.The trick is to make sure there is no 'trap' made by having one part of the branch pipe from the riser main to the radiator have a 'dip' in it, and making sure there is a slope of at least 1 inch in 10 feet in the horizontal with the downward slope away from the radiator, so the condensate can run down to the boiler main without making a pocket of water to slam along the pipes as the system starts and runs. Keeping the connecting branch pipe the same size as now should make it work quietly as there is only a few feet difference (don't use smaller pipe.)